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1. Project report : DNS on Chord
https://github.com/soumyadeep2007/dns chord
Anupam Samanta, Jatin Garg, Soumyadeep Chakraborty
May 2018
1 Abstract
A peer to peer DNS service eliminates the coupling between service and
administration that is inherent in traditional hierarchical DNS. An over-
whelming source of incorrect replies or DNS query failures is borne out of
name-server administrative errors[1, 2]. Implementing such a service on top
of Chord [3], a distributed hash table, not only remedies this problem, but
also confers benefits such as fault-tolerance through replication and load-
balance. [4] explores this idea at considerable depth and we look towards it
as our main source of inspiration. Studies[2] have shown that as much as
18% of overall DNS traffic is towards the root servers. A peer-to-peer system
essentially eliminates any hierarchy whatsoever, and by extension, obviates
the need to have root or TLD servers, providing for better load balance. This
project, through an implementation of DNS on top of Chord, has provided
an opportunity to study the aforementioned aspects. The implementation
is done on top of a research language, DistAlgo [5], which provides powerful
primitives for distributed programming. A comparative performance study
of our DNS service with traditional DNS is performed. For our experiments,
we utilized a sizeable dataset of AAAA records which we obtained from the
Forward DNS dataset [6].
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2. 2 Introduction
2.1 P2P DNS
A P2P DNS system, such as the one we propose is different from a traditional
DNS in these ways:
1. There is no administrative hierarchy coupled to the ownership of records.
Any node can house any record. Records from the same domain may
map to entirely different nodes. Records can be added/removed freely
without having the need to contact any administrative body.
2. Since there is no hierarchy in the system, certain types of DNS records
do not have any value anymore. For example: the NS record is extra-
neous.
3. A P2P DNS can work with both recursive and iterative query reso-
lution. The advantages of a recursive query scheme in a P2P DNS
outweighs the advantages if a similar scheme is adopted in traditional
DNS. In traditional DNS, a recursive query scheme would burden the
servers at the top of the hierarchy, as they receive maximum traffic.
P2P DNSes have no similar hotspots, as they don’t have hierarchy.
Thus a DNS server can readily forward requests to another server in a
P2P scheme.
2.2 Chord
Chord is a distributed lookup protocol, implemented as a distributed hash
table, which addresses a common concern of P2P systems : efficient location
of a node storing an item. The efficiency of Chord is inherent in its guarantee
to answer any query in O(log2N) messages where N is the number of nodes
in the system. The beauty of Chord lies in the fact that this guarantee is
achieved regardless of which node receives the initial query. Chord provides
a simple high-level API for:
1. Mapping a key-value pair to a node.
2. Locating the node to which a particular key is mapped.
3. Querying the node after locating it to obtain the desired key-value pair.
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3. Figure 1: Chord ring having 10 nodes storing 5 keys
2.2.1 Consistent Hashing
Chord relies on Consistent Hashing[7] to assign keys to nodes. In this scheme,
both keys for data and for nodes are hashed to different positions in an
identifier circle, with keys ranging from 0 to 2m
− 1. The circle represents a
key space of 2m
keys. Each position in the ring is represented as an m-bit
identifier. Please note that throughout this report, we have referred to keys
and identifiers interchangeably. Identifiers are obtained as follows:
1. Identifiers for data are obtained by hashing their keys with SHA-1[8].
2. Identifiers for nodes are obtained by hashing their IP addresses with
SHA-1.
To avoid collisions in the assignment of identifiers, techniques such as
Universal Hashing can be adopted.
Consistent hashing works in the following manner: Identifiers are ordered
in the circle modulo 2m
. Key k is assigned to the first node whose identifier
either is equal to k or succeeds k in the identifier circle in the clockwise
direction. Such a node is defined as successor(k). Similarly, predecessor(k)
can be defined. As depicted in Fig. 1, the successor of K10 is N14 since 14
follows 10 in the Chord ring. In general, for a node with identifier i will hold
keys lying in the interval: (predecessor(k), i].
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4. Figure 2: Simple key location: Message sequence for key 54.
2.2.2 Key Location
Depending on the amount of state maintained at each node about its suc-
cessors, the number of messages that need to be sent to locate a key will
scale.
Simple Key Location is a scheme in which each node only stores the
network address of its immediate successor. In the worst case, lookup would
take O(N) messages if the Chord ring has N nodes. The sequence of messages
is depicted for a lookup on the key 54 in Fig 2.
Scalable Key Location is a scheme in which each node stores the network
addresses of O(log2m) ≈ O(log2N) successors, in a data structure called a
finger table. The ith entry in this table at node n will house s = successor(n
+ 2i−1
). Now lookup can be done with a binary search on the table. To
sum up, in the worst case this scheme would incur an overhead of O(log2N)
messages. The finger table for the initial node 8 and message sequence for
resolving the key 54 starting at node 8 is depicted in Fig 3.
2.3 DistAlgo
The language supports distributed programming at a very high level with
high level constructs for: (1) distributed processes, (2) sending and receiving
messages, (3) synchronization conditions expressed as high-level queries over
message history sequences, (4) process and communication configuration.
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5. Figure 3: Scalable key location: a) Finger table at node 8. b) Message
sequence for key 54.
One of the key constructs in the language are the send and receive. Spe-
cific tuples are sent out to specified nodes. Receivers are set up to receive
messages with specific tuple patterns.
3 Approach
3.1 Record format
We only consider DNS AAAA records (which is a hostname and IPv6 address
pair) for purposes of experimentation, given the number of AAAA records
in our dataset was apt. Our experiments could be conducted just as easily
with other DNS record as well.
Format : (’hostname’, ’IPv6 address’)
3.2 DistAlgo setup
We have one DistAlgo process, the main process, setup the Chord nodes
as separate processes and a resolver process. All of these processes execute
on the same physical machine, but can be made to execute across physical
machines via DistAlgo’s multi-host configuration. As a key part of its initial-
ization, each Chord node receives the key-value pairs that it will house along
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6. with its finger table entries. Finger table entries are calculated at the main
process after all nodes have been created, and sent to the nodes upon their
initialization. The resolver, during setup, is fed its workload: a list of DNS
AAAA queries that it has to resolve. We ran DistAlgo over an unreliable
UDP channel.
3.3 Name Resolution
Each process communicates with each other via messages. Fig. 4 depicts a
typical workflow to resolve the key ’kx’ which involves the following messages:
1. The resolver sends a ’find successor’ message to a node picked at ran-
dom. In the example this node is ’i’.
2. The finger table at ’i’ is looked up and it is determined that the next
closest node to ’kx’ is ’m’. So a ’find successor’ message is sent to ’m’
from ’i’.
3. The finger table at ’m’ is looked up and it is determined that the next
closest node to ’kx’ is ’p’. So a ’find successor’ message is sent to ’p’
from ’m’.
4. The finger table at ’p’ is looked up and it is determined that the suc-
cessor for ’kx’ is ’s’. This information is sent across to the resolver from
’p’ in a ’successor’ message.
5. Having received the ’successor’ message the resolver now has located
the node(’s’) where ’kx’ resides. It now sends across a ’get’ message to
’s’ for ’kx’.
6. The node ’s’ responds with a ’result’ message to the resolver which
contains the key-value pair: ’kx’: ’kxv’. Please note: if the key ’kx’
was not part of the ring (i.e. we looked up a DNS name that does not
exist), a null value is returned by ’s’ as part of a ’result’ message.
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7. Figure 4: A typical workflow of messages for resolving the key ’kx’
4 Evaluation
4.1 Experimental setup
DistAlgo can be executed over both reliable (TCP) and unreliable (UDP)
channels via a configuration parameter. We chose to do both and we have
compared their performance.
The Forward DNS dataset was easy to use and we applied it both on our
Chord implementation and traditional DNS with the help of an online Dig
tool[9]. We conducted the experiments with a random subset of these records.
We randomly sampled 10,000 AAAA DNS records from the dataset. These
10,000 records have 8328 2nd level domains, following a zipfian distribution.
4.2 Performance Study
For the performance study, we collected the following metrics: (1) average
query resolution time (2) average number of DNS servers (hops) involved in
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8. resolving a query (3) average network latency and (4) node load distribution.
The online dig tool was configured to bypass the local DNS cache every
time it resolved a query. By enabling the trace option, we could collect the
hops metric as well as measure the average network latency between the dig
tool and each remote DNS server, authoritative or otherwise to arrive at the
average latency metric.
From our Chord implementation, we could easily derive the hops metric,
simply incrementing a counter as a query request travelled across nodes. We
had to simulate network latency in our implementation in order to enable a
fair comparison. This was because the entire Chord ring was executed on
a single physical machine. To simulate this, we inserted a sleep whenever
any process received a message from another process. The sleep duration
was randomly sampled from the Gaussian distribution of network latencies
as gathered from the dig tool.
4.3 Parameters
The parameters that we varied for our Chord implementation are:
1. m
2. N - the number of servers
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9. 4.4 Results
4.4.1 Number of hops
Figure 5: Comparative study of number of hops for N=30.
We define the number of hops with respect to DNS resolution as the number
of DNS servers that are involved in serving a DNS query. We compared the
number of hops metric for our Chord implementation with that of DNS.
Fig 5 shows the number of hops comparison for a N=20, m=20 Chord
ring versus traditional DNS. We can see that traditional DNS outperforms
Chord. While traditional DNS requires 3.86 hops as compared to 4.30 hops
for Chord on average.
If we decrease N, the number of hops for our Chord implementation de-
creases, as expected. This is depicted in Fig 6. This might indicate that
decreasing N is a great way to reduce the number of hops, and in turn, reduce
latency for query resolution. However, there lies the caveat that decreasing
the number of servers would increase the load on each server. We have not
considered the number of DNS records as an experiment parameter. We have
assumed that the cost of looking up a DNS record is negligible as compared
to network latency between two nodes. For our experiments with 10,000
records, such an assumption is viable. However, for a real world setting,
there would be many more records and processing time must be considered.
Not only would processing time at each node increase by reducing N, fault
tolerance would also suffer as number of replicas for each record would also
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10. have to decrease with N. Reducing N would also go against the philosophy
of our system, where we are using commodity servers to serve DNS queries.
To handle larger loads, commodity servers would not be enough.
Figure 6: Effect of N on hop count. Varying m did not make much of an
impact on number of hops. We believe that this has to do with the number of
DNS records that we used. We would have to had more than 10,000 records
to see an impact after varying m.
4.4.2 DNS resolution latency
Figure 7: Comparative study of resolution time for N=30.
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11. We compared the average resolution latency of our Chord implementation
against that of traditional DNS. We modelled real world latency by using a
kernel density estimator on a distribution of network latencies obtained from
making traditional DNS requests. We sampled from the kernel distribution
to inject latency on every distinct network call.
The results were not surprising considering our results of the number of
hops metric. Since, the processing time at each node is negligible given our
small number of DNS records, average resolution latency is directly correlated
with number of hops. Trivially, an increase in the number of hops increases
the number of network calls, and in turn, increases the resolution time. The
results are shown in Figure 7, for m=20, N=30.
Figure 8: Tweaking N to see effect of N on resolution time. Varying N yields
a similar curve as the number of hops, as is expected. Varying m did not
make any impact for reasons similar as above.
4.4.3 Node load distribution
Figure 9 shows the number of requests that a certain node participated in
resolving either yielding an intermediate reply or an authoritative response,
for m=20, N=30. As we can see in the figure, the load is distributed quite
randomly. This is in stark contrast to traditional DNS where the nodes near
the root of the hierarchy are involved in processing each and every query.
This is consistent with what we had set to achieve.
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12. Figure 9: Alternative visualization for node load.
As we increase N, we find that the load per server decreases exponentially,
as expected. We can explain this as follows: The expected number of queries
that a node would participate in (E) = (Average no of hops) * (Number of
requests) / (Number of servers). We can see that as the hop count increases
slowly as we increase N and given our fixed number of requests, the node
load falls rapidly initially but shows a lesser rate of decrease at higher values
of N. Varying m did not make any impact for reasons similar as above.
Figure 10: Alternative visualization for node load.
In Figure 10 x-axis depicts 2LDs sorted on their counts in the dataset.
For example: the 8305th value on the x-axis represents a 2LD: google.com
which has 55 URLs. The y-axis gives the number of nodes which contains
these URLs. We can see that the higher the count of URLs under a TLD,
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13. greater is the number of nodes serving it, thereby achieving a more uniform
distribution.
Figure 11: Effect of N on node load distribution. Varying N has no effect on
the load balance. Varying m did not make any impact for reasons similar as
above.
5 Discussion and Future Work
We found that the number of servers involved in resolving a DNS query in
traditional DNS was far less as compared to our Chord implementation. This
can be attributed to the large branching factor at the root of the DNS hier-
archy that is found in traditional DNS. In our system, we cannot do better
than O(log2N) messages in a Chord ring having N nodes. If we implemented
Pastry or Kademlia which are improvements over Chord, we could have guar-
anteed message latencies around O(log16N). Our work can be extended to:
1. Implement replication of key-value pairs using Chord successor lists, a
feature which replicates key-value pairs belonging to a node in n suc-
cessor nodes. This feature is a large improvement over traditional DNS
where replication is optional throughout the hierarchy and depends on
the resources available to the concerned institution.
2. Implement caching and indexing of DNS records for faster retrieval, a
feature that is consistent in traditional DNS.
3. Implement Chord’s node join and stabilization protocol with which
nodes can be added or removed from the system dynamically. Upon
such events, keys would need to be dynamically reassigned to servers.
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14. 6 Conclusion
We first implemented Chord, a distributed hash table which serves as a
generic P2P lookup service. Then we implemented a DNS on top of Chord,
with support for any DNS record type.
Such a system has advantages such as:
1. Better load balance: This is evident by our experiments where we cal-
culated how many requests each node served. The load distribution
was quite uniform across the nodes irrespective of number of servers in
system. This is an advantage over traditional DNS where the root and
TLD servers have to serve almost every other request.
2. Average resolution latencies: In Chord, as there is no hierarchy, the
hops traversed in order to find authoritative name server can be more
than the traditional DNS server. However the good thing is that DNS
queries can sometimes be resolved in one hop which is not possible in
case of traditional DNS.
3. Proofing from DDoS attacks: Chord is more reliable in preventing DDos
attacks as there is no single server serving an entire domain. So we
cannot bring them down all at once by taking out one server, which is
quite contrary to traditional DNS.
4. Elimination of painful DNS server administration: This is also elimi-
nated in Chord as there is no administrative hierarchy.
5. Commodity servers: Chord can be implemented with commodity hard-
ware as compared to the super-servers required towards the root of a
traditional DNS hierarchy.
With all the features that we suggested in future work, this system of reso-
lution of DNS queries through Chord could prove extremely useful.
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chord
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